1. Field of the Invention
The present invention relates to a multilayer bandpass filter in which a plurality of dielectric layers and a plurality of electrode layers are layered on each other.
2. Description of the Related Art
A conventional high-frequency bandpass filter suitable for reduction in size and cost has been constructed by providing a plurality of LC resonators within a laminate in which dielectric layers and electrode layers are layered on each other.
Such a multilayer bandpass filter is disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447.
The construction of the multilayer bandpass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447 will be described with reference to FIG. 1.
FIG. 1 is a circuit diagram of the multilayer bandpass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447. The multilayer bandpass filter 1 includes an unbalanced input terminal 2, balanced output terminals 3A and 3B, and a bandpass filter section 4 disposed between the unbalanced input terminal 2 and the balanced output terminals 3A and 3B. The bandpass filter section 4 includes three resonators 40, which are each defined as a ½-wavelength resonator 41 with both ends being open and which are arranged side by side. Among the three resonators 40, one resonator 40 arranged at a position closest to the unbalanced input terminal serves as an input resonator 401. The unbalanced input terminal 2 is directly connected to the input resonator 401. Another resonator 40 arranged at a position closest to the balanced output terminals 3A and 3B serves as a ½-wavelength resonator 41A for balanced output. The balanced output terminals 3A and 3B are directly connected to the ½-wavelength resonator 41A for balanced output. The input resonator 401 and a middle resonator 40M are electromagnetically coupled to each other, and the middle resonator 40M and the ½-wavelength resonator 41A for balanced output are also electromagnetically coupled to each other. Further, a capacitor C is disposed between each of the open ends of the three resonators 40 and a ground.
In the multilayer bandpass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447, the λ/2 resonators each having both the open ends are arranged on the same substrate surface for coupling between the resonators so as to define filter characteristics.
In the multilayer bandpass filter disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447, however, a degree of coupling between the resonators can be controlled for proper adjustment only by setting the spacing between the resonators in the substrate surface. When strong coupling is to be obtained, the spacing between the resonators needs to be narrowed. Also, the coupling degree varies due to variation in formation of electrodes, etc. This gives rise to a problem that desired filter characteristics are not obtained.
Further, because the shape of the resonator is determined depending on the frequency of the passband, there has also been a limitation in reducing the size of the filter.
In many cases, the balanced terminal side is connected to, e.g., IC input and output signals in the balanced type. In such a case, impedance needs to be converted between the unbalanced terminal and the balanced terminals. With the structure disclosed in Japanese Unexamined Patent Application Publication No. 2005-45447, however, it is not easy to perform setting and adjustment for proper impedance conversion between both the types of terminals.